This invention relates to high temperature, non-metallic, heat reflecting coatings, and, more particularly, to a fluid, air sprayable, heat reflecting coating mixture, method and coated article with a coating for reflecting heat energy in a frequency range up to about 2.6 microns.
Certain articles are designed for use at elevated temperatures, for example at least about 1400.degree. F., and also are subject to exposure to heat energy. Frequently it is desirable to provide the outer surface portion of such an article with a coating that will reflect heat energy from a metal or ceramic substrate in a selected frequency range. The coating reflects heat energy and thereby reduces heat exposure and heat stressing of the substrate. When the surface portion is intended to experience fluid borne particles, it is desirable to provide the coating with erosion resistance as well. Examples of such articles include components for glass or metal melting furnaces. Other examples are components for power generating apparatus such as gas turbine engines parts, generally made from high temperature superalloys or high temperature ceramics, and designed to operate at temperatures of up to about 2000.degree. F. or more.
One current practice to protect a gas turbine engine article surface from the high temperatures experienced during operation is to apply a ceramic type thermal barrier coating, generally called a TBC, to its outer surface. A commonly used type of TBC is a coating based on zirconia stabilized with yttria, for example about 93 wt. % zirconia stabilized with about 7 wt. % yttria. This general type of TBC has been reported in such U.S. Pat. No. as 4,055,705-Stecura et al. (patented Oct. 25, 1977); U.S. Pat. No. 4,328,285-Siemers et al. (patented May 4, 1982); and Gupta et al. 5,236,745 (patented Aug. 17, 1993). However, such TBC coatings have a relatively rough surface and do not provide adequate heat energy reflection in frequency ranges for certain applications. In addition, application of some TBC type of coating requires use of apparatus having a controlled atmosphere or vacuum.
Another current practice for such protection is to apply to an outer surface portion a multi-layer heat reflecting coating by chemical vapor deposition (CVD). Besides being a relatively expensive type of coating, the size of an article that can be coated by CVD is limited by the size of the equipment in which the CVD is conducted, because of the controlled conditions (vacuum, inert gas, etc.) required for CVD application.